1. Field of the Invention
The present invention relates to an antenna device based on the balanced input system, including, for example, a dipole antenna and a loop antenna.
2. Description of the Related Art
In general, the basic antenna elements include, for example, a dipole antenna and a loop antenna based on the balanced input and a monopole antenna and a helical antenna based on the unbalanced input.
The antenna device based on the balanced input has a structure in which no ground plate is utilized and excitation is effected by the antenna device itself. The antenna device based on the unbalanced input has a structure in which excitation is effected by utilizing a ground plate.
When the antenna device based on the unbalanced input is mounted on a mobile communication instrument, the casing of the communication instrument functions as a ground plate. The ground plate is not an infinite plane. Therefore, an inconvenience arises in that it is necessary to adjust the antenna depending on the shape and the size of the casing.
On the other hand, the antenna device based on the balanced input is scarcely affected by the casing. The antenna device based on the balanced input is advantageous in that the adjustment is less laborious as compared with the antenna device based on the unbalanced input. As for the performance, the antenna device based on the balanced input is advantageous in gain and band width, because the antenna device itself is large as compared with the antenna device based on the unbalanced input.
A large number of suggestions have been hitherto made in order to realize a small size of an antenna device and realize a small size of a communication instrument, including, for example, those having an antenna pattern based on an electrode film formed on a surface of a dielectric substrate (for example, see Japanese Laid-Open Patent Publication Nos. 10-41722, 9-162633, and 10-32413).
The antenna device based on the unbalanced input has been hitherto used as an antenna device for the high frequency zone, because of the following reason.
That is, a 1st stage filter, which is connected to the antenna device, is based on the unbalanced output. Therefore, when the antenna device based on the balanced input is connected to the filter based on the unbalanced output, it is necessary to use a balun as a balanced-unbalanced converter.
When the balun is provided, then the number of parts is increased, and the areal size occupied by the substrate is increased. As a result, a problem arises in that it is impossible to realize a small size of the antenna device which is the basic request.
In other words, in the present circumstances, it is advantageous to use an antenna device based on the unbalanced input in view of the insertion loss and the cost.
However, it is clear that if the antenna device based on the balanced input can be connected to the filter without using the balun, then it is possible to sufficiently exhibit the advantages possessed by the antenna device based on the balanced input, and it is possible to further facilitate the realization of a small size and high performance of the antenna device.
The present invention has been made taking the foregoing problems into consideration, an object of which is to provide an antenna device which makes it possible to appropriately select and perform a balanced input (output) and an unbalanced input (output) for the connection between a filter and an antenna, and which makes it possible to realize a small size and high performance of electronic instruments (including communication instruments) provided with the antenna.
According to the present invention, there is provided an antenna device comprising an antenna section which is based on a balanced input/output; and a filter section in which at least an input/output portion connected to the antenna section is based on the balanced input/output.
That is, the input/output system of the filter section on the side of the antenna section is the balanced input/output system. Therefore, it is possible to provide the antenna device wherein the antenna section, which is connected to the input/output terminal of the filter section, is based on the balanced input system.
As described above, in the antenna device according to the present invention, the balanced input (output) and the unbalanced input (output) can be appropriately selected and performed for the connection between the filter section and the antenna section. Thus, it is possible to realize a small size and high performance of the electronic instrument (including the communication instrument) having the antenna section.
In the antenna device constructed as described above, it is also preferable that the device further comprises a ground electrode for constructing a capacitance together with an open end of the antenna section. In this arrangement, the capacitance, which is formed between the open end of the antenna section and the ground electrode, is added to the capacitance of the parallel resonance circuit obtained by the equivalent transformation of the antenna section. Therefore, assuming that the resonance frequency is identical, it is enough that the parallel resonance circuit has a small inductance. As a result, it is possible to further decrease the length of the antenna section (antenna length). Thus, it is possible to shorten the length of the entire antenna section.
That is, the input/output of the filter section on the side of the antenna section is the balanced input/output. Therefore, it is possible to provide an antenna device wherein the antenna section, which is connected to the input/output terminal of the filter section, is based on the balanced input.
In the antenna device constructed as described above, it is also preferable that the device is composed of a dielectric substrate which includes a large number of stacked dielectric layers and which has at least an input/output terminal and a ground electrode formed on its outer circumferential surface; wherein the filter section includes a plurality of xc2xd wavelength resonator elements of a both ends-open type arranged in parallel to one another in the dielectric substrate respectively; and the antenna section is formed on the dielectric substrate.
In this arrangement, the antenna section may be formed on a surface of the dielectric substrate, or it may be formed at the inside of the dielectric substrate. Alternatively, the antenna section and the filter section may be formed in regions which are two-dimensionally separated from each other on the dielectric substrate.
The xc2xd wavelength resonator, which is used for the filter section as one of the constitutive elements of the antenna device according to the present invention, has such a form that both ends are open. Therefore, it is unnecessary to form the resonator so that it extends up to the end of the dielectric substrate. The resonance frequency is not dispersed depending on, for example, any variation in substrate size caused during the production process. Therefore, it is possible to provide a high performance antenna device.
In the antenna device constructed as described above, it is also preferable that two input/output electrodes, which are arranged at positions of linear symmetry with respect to a center in a length direction of at least the xc2xd wavelength resonator disposed on an output side of the plurality of xc2xd wavelength resonators, are provided in the dielectric substrate; and the two input/output electrodes are connected to balanced input/output terminals of the antenna section.
That is, the filter section can appropriately select and perform the balanced input (output) and the unbalanced input (output) for the connection with the antenna section. Therefore, it is possible to use an antenna based on the balanced input/output system for the antenna section.
As described above, the filter section, which is one of the constitutive elements of the antenna device according to the present invention, makes it possible to obtain the balanced output by obtaining the outputs from the two electrodes disposed at symmetric positions with respect to the middle point of the xc2xd wavelength resonator. On the other hand, when antiphase signals are input at the symmetric positions with respect to the middle point of the xc2xd wavelength resonator, it is possible to cause the resonance. Accordingly, it is possible to perform the balanced input.
In the conventional technique, in order to connect the filter with the antenna element based on the balanced input/output system, it has been necessary to add the balun therebetween. On the contrary, in the present invention, the balanced input (output) and the unbalanced input (output) can be appropriately selected and performed for the connection with the antenna element. Therefore, it is possible to make the connection with the antenna section based on the balanced input/output system without using any excessive circuit part such as the balun. This contributes to the realization of the small size and the high performance of the antenna device.
In the antenna device constructed as described above, it is also preferable that the two input/output electrodes are capacitively coupled to the xc2xd wavelength resonator disposed on the side of the antenna section respectively. Alternatively, it is also preferable that the two input/output electrodes are directly connected to the xc2xd wavelength resonator disposed on the side of the antenna section respectively.
It is also preferable for the antenna device constructed as described above that the filter section includes a coupling-adjusting electrode which is overlapped with the adjoining xc2xd wavelength resonators with the dielectric layer interposed therebetween in the dielectric substrate and which effects capacitive coupling for the adjoining xc2xd wavelength resonators.
Accordingly, the capacitances are formed between the coupling-adjusting electrode and the xc2xd wavelength resonator and between the coupling-adjusting electrode and another xc2xd wavelength resonator respectively. The equivalent circuit has such a form that a combined capacitance of these capacitances is connected in parallel to the inductive coupling formed between the adjoining xc2xd wavelength resonators. Therefore, it is possible to adjust the degree of the coupling by means of the capacitance. Thus, it is possible to obtain the filter having a desired band width.
The capacitance can be easily adjusted by changing the overlapped areal size between the xc2xd wavelength resonator and the coupling-adjusting electrode, the distance therebetween, and/or the permittivity xcex5r of the dielectric disposed therebetween.
Equivalently, the combined capacitance based on the coupling-adjusting electrode is connected in parallel to the inductive coupling between the xc2xd wavelength resonators. Therefore, the parallel resonance circuit is consequently inserted and connected between the adjoining xc2xd wavelength resonators. The impedance of the parallel resonance circuit composed of the capacitance and the inductance is changed from the inductive to the capacitive at lower and higher than the parallel resonant frequency. Accordingly, the coupling between the xc2xd wavelength resonators can be made either inductive or capacitive by adjusting the value of the capacitance formed between the adjoining xc2xd wavelength resonators and the coupling-adjusting electrode respectively.
It is now assumed that the coupling between the xc2xd wavelength resonators is inductive. The parallel resonance point exists on the high frequency side of the pass band. Therefore, it is possible to obtain a filter which has the attenuation pole on the high frequency side. On the other hand, when the coupling between the xc2xd wavelength resonators is capacitive, the parallel resonance point exists on the low frequency side of the pass band. Therefore, it is possible to obtain a filter which has the attenuation pole on the low frequency side. In any case, it is possible to improve the attenuation characteristic of the filter.
In the antenna device constructed as described above, it is also preferable that a plurality of coupling-adjusting electrodes as defined above are formed; and the plurality of coupling-adjusting electrodes are formed at positions of linear symmetry with respect to a center in a length direction of the xc2xd wavelength resonator.
In this arrangement, it is possible to suppress the influence of the positional discrepancy between the xc2xd wavelength resonator and the coupling-adjusting electrode during the production steps. Specifically, the effect of the coupling-adjusting electrode is affected by the relative position with respect to the xc2xd wavelength resonator. However, when the coupling-adjusting electrodes are formed at the positions of linear symmetry with respect to the center in the length direction of the xc2xd wavelength resonator, the variations of effects of the plurality of coupling-adjusting electrodes are offset with each other, even if the positional discrepancy occurs in the longitudinal direction of the xc2xd wavelength resonator. Thus, it is possible to suppress the influence of the positional discrepancy between the xc2xd wavelength resonator and the coupling-adjusting electrode.
In the antenna device constructed as described above, it is also preferable that the filter section includes inner layer ground electrodes which are arranged to overlap both open ends of each of the xc2xd wavelength resonators with the dielectric layer interposed therebetween.
In this arrangement, the capacitance, which is formed between the inner layer ground electrode and the open end side of each of the xc2xd wavelength resonators, is also added to the capacitance of the parallel resonance circuit obtained by the equivalent transformation of the xc2xd wavelength resonator. Therefore, assuming that the resonance frequency is identical, it is enough that the inductance of the parallel resonance circuit is small. As a result, it is possible to further decrease the length of the xc2xd wavelength resonator (resonator length). Thus, it is possible to shorten the length of the entire filter section.
In this case, the following problem may arise. That is, when the opposing areal size of the inner layer ground electrode and each of the xc2xd wavelength resonators is increased in order to realize a small size of the filter section, the inductive coupling between the xc2xd wavelength resonators becomes stronger, resulting in a band width that is too broad. However, in the present invention, the coupling-adjusting electrode is provided as described above. Therefore, owing to the capacitance formed between the coupling-adjusting electrode and the xc2xd wavelength resonator, the absolute value of the total susceptance, which is formed by the capacitance between the xc2xd wavelength resonators and the inductive coupling between the xc2xd wavelength resonators, is changed. Therefore, the degree of the coupling between the xc2xd wavelength resonators can be adjusted by adjusting the value of the capacitance. Thus, it is possible to obtain the filter having a desired band width.
Even when any stacking discrepancy occurs in the longitudinal direction (axial direction) of the xc2xd wavelength resonator concerning the xc2xd wavelength resonator and the inner layer ground electrode, it is possible to decrease the dispersion of the resonance frequency, because the changes in capacitance at the respective open ends of the xc2xd wavelength resonators are offset with each other.
In the antenna device constructed as described above, it is also preferable that the dielectric substrate is formed such that a dielectric constant of the dielectric layer on which the antenna section is formed is different from a dielectric constant of the dielectric layer on which the filter section is formed. Especially, when the dielectric constant of the dielectric layer on which the antenna section is formed is lower than the dielectric constant of the dielectric layer on which the filter section is formed, it is possible to realize the small size of the filter section. Simultaneously, it is possible to effectively suppress the low gain and the decrease in band width in the antenna section.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.